The present invention relates to a balancing circuit for use with a digital length or angle measuring system of the type which includes a measuring scale, a scanning unit including at least one scanning element positioned to scan the scale and to generate in response thereto a scanning signal, and an evaluating circuit coupled to the at least one scanning element to evaluate the scanning signal.
Length or angle measuring systems which include arrangements for balancing or correcting faulty signal parameters of scanning signals are known to the art. Such signal parameters which may require balancing include, for example, symmetry, amplitude, and phase angle.
In German DE-OS No. 27 29 697, there is described a measuring system which includes a correction arrangement for the correction of faulty signal parameters of scanning signals which are to be interpolated. Deviations in the desired signal parameters are stored as digital correction values in a memory, and a computer utilizes these stored digital correction values to correct digitized scanning signals prior to interpolation. This correction system is relatively complicated in structure and use, and it requires the use of a computer and a memory which are not always available in length or angle measuring systems.
German DE-OS No. 28 18 742 describes a length measuring system in which a scanning unit generates two scanning signals which are phase shifted with respect to one another. In order to allow adjustment of the desired mutual phase shift between the scanning signals, one scanning signal is applied to a first input of an amplifier mixing stage. The second input of this amplifier receives a regulating voltage which is derived from the other scanning signal and is adjustable by means of an operational amplifier and a potentiometer. This arrangement is suited only for balancing the phase relationship between the two scanning signals and it requires the use of an amplifying element in the form of an operational amplifier.
German DE-OS No. 30 07 311 discloses a length or angle measuring system in which at least one pair of opposed phase photosensors is provided for generating scanning signals. These photosensors are connected in an anti-parallel arrangement to a pulse forming stage of an external evaluating arrangement. A constant balancing signal is applied to the circuit of the photo elements in the scanning unit in order to allow symmetry balancing of the counterphase scanning signals. In this system, only the symmetry of opposed phased scanning signals can be balanced. Furthermore, the symmetry balancing occurs only in one direction.